Field of the Invention
This invention concerns a digital copying apparatus in which an image is read from an original, and converted to electric signals, which are supplied in binary form to a recording device.
In a digital copying apparatus, an image to be copied is generally obtained by reading an image on every minute region of an original, that is, every picture element by using, for example, a CCD (charge-coupled device) image sensor, converting electric analog signals issued at the output of the image sensor into digital signals (analog to digital conversion), applying various kinds of processings to the thus obtained digital signals and then supplying dot signals to a recording device.
By the way, in the recording device used in the copying apparatus of this type, since it is difficult to change the density level on the picture elements to be recorded, the recording of each picture element is generally carried out in binary form. However, since an original document may sometime include intermediate tone images or half tone images such as in the case of a photograph, it is necessary to reproduce an image with the intermediate tone. For recording the intermediate tone by using a recording device that conducts binary recording, there have been proposed various methods such as dither method, density pattern method and submatrix method. The image with the intermediate tone can be reproduced by using any of these methods.
However, in such a method, while a relatively fine copy is available where the density of the original image changes gradually as in photographic image, contours of the copy image become indistinct making letters unclear. Faint contaminations in the background appear on the copied image to deteriorate the copy quality where a line image (highlight and shadow) is to be processed. This is why a switch is provided in such a machine by which, when a line image is processed, half tone processing is omitted and the signal is simply binarized. This switch is turned on or off by an operator, who judges the appropriate mode according to the type of the originals.
However, it is common that both a line image with binary tone and gradation image with continuous tone are present in a single document, as in a brochure. In such a case, the binary mode produces poor photographs, while letters are unclear with the gradation mode.
There is another problem in a digital copying apparatus of this type. The image is usually read as a small picture element with a line sensor. If there is a systematic change in density, moire fringes may sometimes appear, due to the interference of the two. A number of image reading sensors are arranged with some pitch, while the density may change in a comparable order of frequency. For instance, photographs are often mesh-printed with regular pitches between the meshes, which are likely to give moire fringes. When 16 image reading sensors are placed in a millimeter, moire fringes are liable if the mesh density is close to that resolution power, that is, from 133 lines (about 10.5 meshes/mm) to 200 lines (16). Moire fringes, of course, may also appear at other densities but at the afore-mentioned density the signals fluctuate by a large amount.
The mesh-printing itself is a sort of simulated expression for continuous tone, in which the density variation at a particular picture element is in a 1/0 (record/not-record) binary fashion. In mesh-printing, a group of picture elements changes as a whole, depending on the pitch of the meshes or the size of the mesh point, thereby reproducing a continuous tone. Accordingly, when a mesh-printed original image is to be copied on a binary machine, moire fringes are liable at a specific density, deteriorating the copy quality.
This problem is less serious when the data is converted into a binary code signal with half tone processing, like averaging the density of a plurality of picture elements, changing the level of the threshold value, etc. In this case, the copied image is also simulated as a group of mesh points. However, they are not a direct reproduction of those on the original: the mesh points being newly formed by the half tone processing.
Accordingly, a mesh-printed or mesh-copied original is better processed in a gradation mode with half tone processing, although picture elements are recorded in a binary fashion.